1. Field of the Invention
The invention relates to a mechanical, straight-line, four bar lift mechanism suitable for use on a conveyor assembly line wherein workpieces, transported along a conveyor, are gently lifted vertically, in a straight-line, from the conveyor while the mechanism moves through a dwell and then rapidly accelerates upwardly to an overhead work station where the linkage reaches a dwell such that work may be performed upon the workpiece and, thereafter, the workpiece is returned to the conveyor and the mechanism continues to travel below the conveyor to a dwell while in an at rest position.
2. Description of the Prior Art
Straight-line linkage mechanisms are well known for transferring workpieces into and out of a press or machine and are most advantageous in such applications because of the accuracy of positioning the workpieces at the beginning and end of travel of the respective mechanism. For example, U.S. Pat. No. 4,295,780 teaches a straight-line linkage apparatus for automatically transferring workpieces which is characterized by extremely consistent movements during the starting and stopping stages thereof as well as by accurate stop positions thereof. This characteristic is provided by the fact that the velocity characteristics of the linkage and crank mechanism closely resembles that of a sine curve, which curve has longer or smoother foot portions to thereby allow smooth starting and stopping motion of the linkage mechanism. In fact, the linkage approaches a dwell at the start and stop positions.
Another widely used straight-line linkage mechanism is the so called Scott-Russell mechanism wherein an oscillating link in combination with a plurality of intermediate links is arranged to provide an approximate straight-line travel of the output link.
The above straight-line mechanisms are often criticized because of their instability where there is side loading on the mechanism in a direction transverse to the generated straight-line as the links travel through their respective arcuate motions to provide a straight-line output to the output links. These mechanisms are also generally bulky since the links must pass over each other making it difficult to provide a construction which is capable of reacting to transverse loads.
U.S. Pat. No. 4,545,266 to Brems attempts to resolve some of the shortcomings of the above described problems associated with straight-line linkage mechanisms. Brems teaches a mechanical linkage system for generating a straight-line utilizing links which do not cross over in the motion generated. The disclosed linkage teaches a rigid bell crank member which has a center pivot connection to a first suspension link, a second pivot connection to a substantially linear guide mechanism and a third pivot connection to an output point, the dimensions of the link, the distance between the outer pivot connection and the guide mechanism, and between the center pivot and the output point are generally equal allowing the mechanism to be used as a linear slide or a straight-line linkage.
Although such mechanical linkage overcomes the shortcomings of the Scott-Russell mechanisms, there are still numerous problems associated with the Brems straight-line linkage as utilized in wide applications. For example, such straight-line linkage is utilized in a welding press in U.S. Pat. No. 4,600,095 to Brems to elevate a platen from a rest position spaced below a conveyor to an elevated position and during this straight-line movement a workpiece is picked up from the conveyor by the lower tooling attached to the platen and moved to an upper position against tooling attached to the upper portion of the press to perform welding operations to the workpiece. To utilize the Brems straight-line linkage additional structure is required in the base of the welding press in order to accommodate the sliding pivot support. Further, the larger links that are required to be of equal length to make the straight-line mechanism operate according to the teachings of the invention occupy significantly more space than a Scott-Russell mechanism as well as demand special considerations in designing a counterbalance which resides within the confines of the platen. Because of the limitations of the attachment points to the platen, the side thrust capabilities of the welding knee are further limited. Finally, and most important, the straight-line linkage apparatus completely disregards the velocity of the input and output attachment links, resulting in the output and input links at the beginning and end limits of travel moving at relatively high velocity, requiring significant compensation in the parameters of the associated cycloidal drive in order to account for the high inertia forces at the uppermost and lowermost position of the platen. The bulkiness of the linkage, limitation of side thrust capabilities, and associated inertia forces of not only the links but the weight of the tooling carried by the platen (up to 5 tons) has resulted, at times, in unwanted vibration and instability of the mechanical welding knee.
Accordingly, what is needed is a mechanical welding press which provides desired lift characteristics without forfeiting optimum lift kinematic characteristics in favor of the straight-line kinematic requirements. Such a mechanical linkage system would produce a motion approaching a dwell at the beginning and ending limits of its motion as well as provide an intermediate dwell, variable as a function of time so that a workpiece may be elevated on a platen of a welding press with a straight-line vertical motion in two successive steps to first engage a workpiece on a conveyor located between the uppermost and lowermost position of travel and then elevate the workpiece to a work station above the conveyor. Thereafter, the mechanical linkage system lowers the platen with a straight-line vertical motion in two successive steps to first deposit the workpiece on the conveyor and then return to its starting position. The horizontal location of the conveying system will vary and accordingly the mechanism must be adjustable to variation of the midpoint pick up. For example, Rasenberger, U.S. Pat. No. 4,416,198, teaches a drive for producing motion for a printing press with intervening dwells including a four bar linkage defining a couple curve travel path traversable only in part and having equally coincident forward and return paths, and a dwell-producing driven rocker arm pair connected thereto, the couple curve travel path having a vertex therein dividing the couple curve travel path into a short curve path for producing a dwell and an elongated curve path extending at an angle to the short curve path for producing a movement, the vertex of the couple curve being identical with one instantaneous pole defining the start and end of the dwell. In view of the requirements outlined above with respect to a mechanical welding press, the teachings of this drive for producing dwells are not suited for a mechanical welding press application. For example, the linkage as described acquires only one dwell from the four bar linkage, the other dwell being generated by a harmonic input drive. The extended dwell created in the four bar linkage is a function of the position of the links not a velocity function. Further, the dwell apparatus is taught to be run with a single direction drive while a reversing drive is essential for a four bar linkage application having a dwell generated at both ends of travel with a variably occurring dwell therebetween, as will be made clear hereinafter. Finally, the drive for producing motion with dwells as taught in Rasenberger is very complex.
Brems, U.S. Pat. No. 3,789,676, teaches a reciprocating drive mechanism for transmitting an output motion with predetermined acceleration and velocity characteristics which provides a dwell at the beginning and end of its travel as well as a dwell timed precisely at the midpoint of total time traveled between the beginning and end position of its travel. This is accomplished by a lift mechanism powered by a reversible electric motor through a gear box. The output shaft of the gear box has a crank mounted thereon on which is located a fixed gear. A gear rack meshes with the fixed gear and is retained in engagement therewith by a bracket that is journalled on the axis of the fixed gear by a shaft. The pitch line of the teeth of the fixed gear passes through the axis of an output shaft. This arrangement generates a cycloidal motion to the gear rack which is shown in prior art FIG. 13.
The problem associated with utilizing a cycloidal drive to provide dwells at both ends of travel as well as at its midpoint is that the links associated with this type of drive provide a harmonic dwell at at least one end (beginning or ending) of its travel which is required to lock up the platen in order to counteract the downward force generated by the tooling mounted in the welding press as it performs work on the workpiece. Further, the cycloidal drive, as stated above, generates a dwell at both ends of travel and at the precise midpoint in time of the total time to travel through the complete motion of the links. The displacement of the platen to reach the midpoint position in time is therefore very limited since the pickup position for the workpiece must be at the precise midpoint of the total time traveled during the total stroke of the platen, offering little flexibility as to the positioning of the conveyor that travels through the mechanical welding press due to the acceleration and velocity characteristics as a consequence of the pickup position being at exactly the midpoint, in time, of the total time requirement of the complete up and down movement of the platen.
Finally, if the drive is taken to its respective end of travel to its full dwell position, the drive will have infinite power multiplication at the respective end point dwell position, requiring some form of break-away output device as a safety feature to protect the mechanical welding press. Accordingly, what is needed is a four bar linkage system wherein the midpoint dwell of a cycloidal drive is advantageously used to pick up a workpiece from a conveyor system associated with a mechanical welding press and the end point dwells are generated by the four bar linkage itself rather than the cycloidal drive in order to avoid the additional safety devices associated with the end point dwells of the cycloidal drive as well as to offer adjustability for the occurrence of an intermediate dwell so that the stroke of the platen may be adjustable and the workpieces may be picked up from a conveyor which traverses the welding press between a predetermined range of distances from the bottom dwell position of the platen. Such an arrangement eliminates the potential hazards associated with the cycloidal drive end point dwells and permits a range of midpoint pickups of the workpiece from the conveyor by convenient adjustments of the four bar linkage and associated links between the four bar linkage and the cycloidal drive. Further, because of the linkage system's application to a welding press, the linkage must be able to move approximately 5 ton through a stroke of 18 to 24 inches in less than 2 seconds without vibrations or instability. The lift mechanism should be relatively compact as it must necessarily be positioned below the conveyor. The lift mechanism must be structurally rigid in the lateral, longitudinal, and vertical directions to resist the forces imposed by the work performed on the workpiece and imparted to the lift mechanism by the particular operations performed on the workpiece when the lift mechanism is in the raised position. Naturally, it is desired that the lift mechanism rise rapidly to the conveyor location, gently engage and pick up the workpiece and thereafter again rise rapidly to the end of its upper limit of travel to engage the tooling so that work may be performed on the workpiece. Upon completion of the work operation, the lift mechanism must rapidly descend to the conveyor level, gently deposit the workpiece thereon and descend to its rest position below the conveyor thereby permitting the conveyor to index to the next workpiece whereafter the cycle is repeated.